The BDC is set for M855 ballistics, “good enough” whether fired from an M4 or M249.
M4 is capable of 500m point - 600m area engagements.
M249 is capable of area target suppression at greater range than M4 because it is a light machine gun providing greater volumes of fire.
I thought I had an ACOG TM laying around somewhere for the “book” explanation. In any case, the scope is designed for use with either system